Thin film transistor

ABSTRACT

A thin film transistor includes a source electrode, a drain electrode, a semiconductor layer, a channel and a gate electrode. The drain electrode is spaced from the source electrode. The gate electrode is insulated from the source electrode, the drain electrode, and the semiconducting layer by an insulating layer. The channel includes a plurality of carbon nanotube wires, one end of each carbon nanotube wire is connected to the source electrode, and opposite end of each the carbon nanotube wire is connected to the drain electrode.

RELATED APPLICATIONS

This application is related to application Ser. No. 12/384,245,entitled, “METHOD FOR MAKING THIN FILM TRANSISTOR”, filed on Apr. 2,2009; Ser. No. 12/384,331, entitled, “METHOD FOR MAKING THIN FILMTRANSISTOR”, filed on Apr. 2, 2009; Ser. No. 12/384,309, entitled, “THINFILM TRANSISTOR”, filed on Apr. 2, 2009; Ser. No. 12/384,310, entitled,“METHOD FOR MAKING THIN FILM TRANSISTOR”, filed on Apr. 2, 2009; Ser.No. 12/384,244, entitled, “THIN FILM TRANSISTOR PANEL”, filed on Apr. 2,2009; Ser. No. 12/384,329, entitled, “THIN FILM TRANSISTOR”, filed onApr. 2, 2009; Ser. No. 12/384,299, entitled, “THIN FILM TRANSISTOR”,filed on Apr. 2, 2009; Ser. No. 12/384,292, entitled, “THIN FILMTRANSISTOR”, filed on Apr. 2, 2009; Ser. No. 12/384,293, entitled, “THINFILM TRANSISTOR”, filed on Apr. 2, 2009; Ser. No. 12/384,330, entitled“THIN FILM TRANSISTOR”, filed on Apr. 2, 2009; Ser. No. 12/384,241,entitled, “METHOD FOR MAKING THIN FILM TRANSISTOR”, filed on Apr. 2,2009; and Ser. No. 12/384,238, entitled, “THIN FILM TRANSISTOR”, filedon Apr. 2, 2009. The disclosures of the above-identified applicationsare incorporated herein by reference.

BACKGROUND

1. Technical Field

The present invention relates to thin film transistors and,particularly, to a carbon nanotube based thin film transistor.

2. Discussion of Related Art

A typical thin film transistor (TFT) is made of a substrate, a gateelectrode, an insulation layer, a drain electrode, a source electrode,and a semiconductor layer. The thin film transistor performs as a switchby modulating an amount of carriers accumulated at an interface betweenthe insulation layer and the semiconducting layer.

Generally, the material of the semiconductor layer is amorphous silicone(a-Si), poly-silicone (p-Si), or organic semiconducting material. Thecarrier mobility of an a-Si TFT is lower than a p-Si TFT. However, themethod for making the p-Si TFT is complicated and has a high cost. Theorganic TFT has the virtue of being flexible but has low carriermobility.

Carbon nanotubes (CNTs) are a novel carbonaceous material and havereceived a great deal of interest since the early 1990s. Carbonnanotubes have interesting and potentially useful heat conducting,electrical conducting, and mechanical properties. Further, there are twokinds of carbon nanotubes: metallic carbon nanotubes and semiconductingcarbon nanotubes determined by the arrangement of the carbon atomstherein. The carrier mobility of semiconducting carbon nanotubes along alength direction can reach about 1000 to 1500 cm²V⁻¹ s⁻¹. Thus, a TFTemploying a semiconductor layer adopting carbon nanotubes has beenproduced.

However, the carbon nanotubes in the conventional TFT are distributed asa disordered carbon nanotube layer or perpendicular to the substrate asa carbon nanotube array. In the disordered carbon nanotube layer, due todisordered arrangement of the carbon nanotubes, the paths for carriersto travel are relatively long resulting in low carrier mobility.Further, the disordered carbon nanotube layer is formed by printing amixture of a solvent with the carbon nanotubes dispersed therein on thesubstrate. The carbon nanotubes in the disordered carbon nanotube layerare joined or combined to each other by an adhesive agent. Thus, thedisordered carbon nanotube layer has a hardened structure and is notsuitable for being used in a flexible TFT.

In the carbon nanotube array, the carbon nanotubes are perpendicular tothe substrate. However, although the carbon nanotubes have good carriermobility along the length direction, the carrier mobility of the carbonnanotube array along a direction parallel to the substrate is relativelylow.

In sum, the two kinds of carbon nanotube structure employed in theconventional TFT have low carrier mobility and poor flexibility.

What is needed, therefore, is a TFT in which the above problems areeliminated or at least alleviated.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the present thin film transistor can be betterunderstood with references to the following drawings. The components inthe drawings are not necessarily drawn to scale, the emphasis insteadbeing placed upon clearly illustrating the principles of the presentthin film transistor.

FIG. 1 is a cross sectional view of a thin film transistor in accordancewith a first embodiment.

FIG. 2 is a structural schematic of a carbon nanotube segment.

FIG. 3 shows a Scanning Electron Microscope (SEM) image of an untwistedcarbon nanotube wire used in the thin film transistor of FIG. 1.

FIG. 4 shows a Scanning Electron Microscope (SEM) image of a twistedcarbon nanotube wire used in the thin film transistor of FIG. 1.

FIG. 5 is a schematic view of the thin film transistor of FIG. 1connected to a circuit.

FIG. 6 is a view of a thin film transistor in accordance with a secondembodiment.

FIG. 7 is a cross sectional view of a thin film transistor in accordancewith a second embodiment.

Corresponding reference characters indicate corresponding partsthroughout the several views. The exemplifications set out hereinillustrate at least one embodiment of the present thin film transistor,in at least one form, and such exemplifications are not to be construedas limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

References will now be made to the drawings to describe, in detail,embodiments of the present thin film transistor.

Referring to FIG. 1, a thin film transistor 10 is provided in a firstembodiment, and has a top gate structure. The thin film transistor 10includes a semiconductor layer 140, a source electrode 151, a drainelectrode 152, an insulating layer 130, and a gate electrode 120. Thethin film transistor 10 is located on an insulating substrate 110. Theinsulating substrate 110 is provided for supporting the thin filmtransistor 10. The insulating substrate 110 can be a substrate employedin a printed circuit board (PCB). Alternatively, the insulatingsubstrate 10 can be made of rigid materials (e.g., p-type or n-typesilicon, silicon with an silicon dioxide layer formed thereon, crystal,crystal with a oxide layer formed thereon), or flexible materials (e.g.,plastic or resin). In the present embodiment, the material of theinsulating substrate is glass. The shape and size of the insulatingsubstrate 110 are arbitrary. A plurality of thin film transistors 10 canbe assembled on a single insulating substrate 110 according to designneeds.

The semiconducting layer 140 is located on the insulating substrate 110.The source electrode 151 is spaced from the drain electrode 152. Boththe source electrode 151 and the drain electrode 152 are connected tothe semiconducting layer 140. The insulating layer 130 is locatedbetween the semiconducting layer 140 and the gate electrode 120. Theinsulating layer 130 is located between the semiconducting layer 140 andthe gate electrode 120. The insulating layer 130 is located on portionof the semiconducting layer 140, or covers the semiconducting layer 140,the source electrode 151, and the drain electrode 152. The gateelectrode 120 is located on the insulating layer 130. The insulatinglayer 130 is configured to provide the electric insulation between thesemiconducting layer 140, the source electrode 151, and the drainelectrode 152. A channel 156 is part of the semiconducting layer 140 andextends between the source electrode 151 and the drain electrode 152.

The source electrode 151 and the drain electrode 152 can be located onthe semiconducting layer 140 or on the insulating substrate 110. Morespecifically, the source electrode 151 and the drain electrode 152 canbe located on a top surface of the semiconducting layer 140, and locatedat the same side of the semiconducting layer 140 as the gate electrode120. In other embodiments, the source electrode 151 and the drainelectrode 152 can be located on the insulating substrate 110 and coveredby the semiconducting layer 140 (not shown). The source electrode 151and the drain electrode 152 are located on different sides of thesemiconducting layer 140 from the gate electrode 120. In otherembodiments, the source electrode 151 and the drain electrode 152 can beformed on the insulating substrate 110, and coplanar with thesemiconducting layer 140.

The semiconducting layer 140 includes a plurality of carbon nanotubewires. Opposite ends of at least some of the plurality of carbonnanotube wires are connected to the source electrode 151 and the drainelectrode 152. The arrangement of the carbon nanotube wires can varyaccording to the practice requirements. The carbon nanotube wires canparallel to each other or cross with each other. In this embodiment, thecarbon nanotube wires are parallel to and in contact with each other,and the carbon nanotube wires are aligned along a direction extendingfrom the source electrode 151 to the drain electrode 152.

The carbon nanotube wire can have a twisted structure or an untwistedstructure. Referring to FIGS. 2 and 3, the untwisted carbon nanotubewire includes a plurality of successively oriented carbon nanotubesegments 143 joined end-to-end by van der Waals attractive force. Eachcarbon nanotube segment 143 includes a plurality of carbon nanotubes 145parallel to each other, and combined by van der Waals attractive force.The carbon nanotube segments 143 can vary in width, thickness,uniformity and shape. The carbon nanotubes in the untwisted carbonnanotube wire are primarily oriented along one direction (i.e., adirection along the length of the wire). Referring to FIG. 4, thetwisted carbon nanotube wire includes a plurality of carbon nanotubesoriented around an axial direction of the carbon nanotube wire. Morespecifically, the carbon nanotube wire includes a plurality ofsuccessive carbon nanotubes joined end to end by van der Waalsattractive force. A length and a diameter of the carbon nanotube wirecan be set as desired. In the present embodiment, a diameter of thecarbon nanotube wire is in a range from about 0.5 nanometers to about100 micrometers (em). A distance between the carbon nanotube wires canrange from 0 to about 1 millimeter. The carbon nanotubes of the carbonnanotube wire can be semiconducting carbon nanotubes, and can beselected from a group consisting of single-walled carbon nanotubes,double-walled carbon nanotubes, or combination thereof. The diameter ofthe single-walled carbon nanotube is in a range from about 0.5nanometers to about 50 nanometers. The diameter of the double-walledcarbon nanotube is in a range from about 1 nanometer to about 50nanometers. In the present embodiment, the diameter of thesemiconducting carbon nanotubes is less than 10 nanometers.

It is to be understood that the carbon nanotube wire provides superiortoughness, high mechanical strength, and is easy to bend. As such, thesemiconducting layer 140 of the present embodiment can be used with aflexible substrate to form a flexible TFT.

A length of the semiconducting layer 140 can be in a range from about 1micrometer to about 100 micrometers. A width of the semiconducting layer140 can be in a range from about 1 micrometer to about 1 millimeter. Athickness of the semiconducting layer 140 can be in a range from about0.5 nanometers to about 100 microns. A length of the channel 156 can bein a range from about 1 micron to about 100 microns. A width of thechannel 156 (i.e., a distance from the source electrode to the drainelectrode) can be in a range from about 1 micrometer to about 1millimeter. In the present embodiment, the length of the semiconductinglayer 140 is about 50 micrometers, the width of the semiconducting layeris about 300 micrometers, the thickness of the semiconducting layer 140is about 25 nanometers, the length of the channel 156 is about 40microns, and the width of the channel 156 is about 300 microns.

The carbon nanotube wires of the semiconducting layer 140 can be acarbon nanotube structure that has been treated. The possible carbonnanotube structures include a carbon nanotube film. The carbon nanotubefilm is fabricated by being drawn from a carbon nanotube array. Thecarbon nanotube film can be twisted to form the carbon nanotube wires.The carbon nanotube wires are adhesive due to a large specific surfacearea of the carbon nanotubes and the high purity of the carbon nanotubestructure. Thus, the plurality of carbon nanotube wires can be placedand adhered on the insulating substrate 110 directly. In addition, inorder to enhance the adhesion force, an adhesive can be used to adherethe carbon nanotube wires to the insulating substrate 110. Morespecifically, the plurality of carbon nanotube wires can be adhered onthe substrate 110 in advance of providing the source electrode 151 andthe drain electrode 152. Then, the source electrode 151 and the drainelectrode 152 are located along the direction of the carbon nanotubes ofthe carbon nanotube wires. Alternatively, the source electrode 151 andthe drain electrode 152 can be formed on the substrate 110 in advance ofproviding the carbon nanotube wires. Then, the carbon nanotube wires areadhered to the insulating substrate 110 along the direction from thesource electrode 151 to the drain electrode 152. In such case, portionof the carbon nanotube wires can be placed on the source electrode 151and the drain electrode 152.

In the present embodiment, the source electrode 151 and the drainelectrode 152 are spaced from each other and located on the oppositeends of the carbon nanotube wires. In addition, the source electrode 151and the drain electrode 152 are connected to the carbon nanotube wires.In the present embodiment, the distance between the source electrode 151and the drain electrode 152 is in a range from about 1 micrometer toabout 100 micrometers.

The source electrode 151, the drain electrode 152, and/or the gateelectrode 120 can be made of conductive material. In the presentembodiment, the source electrode 151, the drain electrode 152, and thegate electrode 120 are conductive films. A thickness of the conductivefilm can be in a range from about 0.5 nanometers to about 100micrometers. The material of the source electrode 151, the drainelectrode 152, and the gate electrode 120 can be selected from the groupconsisting of metal, alloy, indium tin oxide (ITO), antimony tin oxide(ATO), silver paste, conductive polymer, or metallic carbon nanotubes.The metal or alloy can be selected from the group consisting of aluminum(Al), copper (Cu), tungsten (W), molybdenum (Mo), gold (Au), titanium(Ti), neodymium (Nd), palladium (Pd), cesium (Cs), and combinations ofthe above-mentioned metal. In the present embodiment, the sourceelectrode 151, the drain electrode 152, and the gate electrode 120 arePd films. A thickness of the Pd film is about 5 nanometers. The Pd filmshave a good wettability.

The material of the insulating layer 130 can be a rigid material such assilicon nitride (Si₃N₄), silicon dioxide (SiO₂), or a flexible materialsuch as polyethylene terephthalate (PET), benzocyclobutenes (BCB),polyester or acrylic resins. A thickness of the insulating layer 130 canbe in a range from about 5 nanometers to about 100 microns. In thepresent embodiment, the insulating layer 130 is made from Si₃N₄.

Referring to FIG. 5, in use, the source electrode 151 is grounded. Avoltage Vds is applied to the drain electrode 152. Another voltage Vg isapplied on the gate electrode 120. The voltage Vg forming an electricfield in the channel 156 of the semiconducting layer 140. Accordingly,carriers exist in the channel 156. As the Vg increases, a current isgenerated and flows through the channel 156. Thus, the source electrode151 and the drain electrode 152 are electrically connected. The carriermobility of the semiconducting carbon nanotubes along the lengthdirection thereof is relatively high, and the carbon nanotubes of thecarbon nanotube wire are aligned substantially from the source electrode151 to the drain electrode 152. Therefore, the paths for the carriers totravel in the semiconducting layer 140 are short, causing high carriermobility. In the present embodiment, the carrier mobility of the thinfilm transistor 10 is higher than 10 cm²/V⁻¹ s⁻¹ (e.g., 10 to 1500cm²/V⁻¹ s⁻¹), and the on/off current ratio of the thin film transistor10 is in a range from about 1×10² to about 1×10⁶.

Referring to FIGS. 6 and 7, a thin film transistor 20 is provided in asecond embodiment and has a bottom gate structure. The thin filmtransistor 20 includes a gate electrode 220, an insulating layer 230, asemiconducting layer 240, a source electrode 251, and a drain electrode252. The thin film transistor 20 is located on an insulating substrate210.

The compositions, features and functions of the thin film transistor 20in the second embodiment are similar to the thin film transistor 10 inthe first embodiment. The difference is that, the gate electrode 220 ofthe second embodiment is located on the insulating substrate 210. Theinsulating layer 230 covers the gate electrode 220. The semiconductinglayer 240 is located on the insulating layer 230, and insulated from thegate electrode 220 by the insulating layer 230. The source electrode 251and the drain electrode 252 are spaced apart from each other andconnected to the semiconducting layer 240. The source electrode 251, andthe drain electrode 252 are insulated from the gate electrode 220 by theinsulating layer 230. A channel 256 is formed in the semiconductinglayer 240 at a region between the source electrode 251 and the drainelectrode 252.

The source electrode 251 and the drain electrode 252 can be located onthe semiconducting layer 240 or on the insulating layer 230. Morespecifically, the source electrode 251 and the drain electrode 252 canbe located on a top surface of the semiconducting layer 240, and at thesame side of the semiconducting layer 240 with the gate electrode 220.In other embodiments, the source electrode 251 and the drain electrode252 can be located on the insulating layer 230 and covered by thesemiconducting layer 240. The source electrode 251 and the drainelectrode 252 are on different sides of the semiconducting layer 240from the gate electrode 220. In other embodiments, the source electrode251 and the drain electrode 252 can be formed on the insulating layer230, and coplanar with the semiconducting layer 240. The semiconductinglayer 240 includes a plurality of carbon nanotube wires 260.

The thin film transistors provided in the present embodiments have atleast the following superior properties. The carbon nanotube wires aretough and flexible. Thus, thin film transistors using metallic carbonnanotube wires as electrodes can be durable and flexible. The carbonnanotube wires are durable at high temperatures. Therefore, the thinfilm transistor using carbon nanotube wires as the semiconducting layercan be used in high temperature. The thermal conductivity of the carbonnanotube wires is relatively high. Thus, in use, heat produced by thethin film transistor can be rapidly spread out and easily dissipated.

It is to be understood that the above-described embodiments are intendedto illustrate rather than limit the invention. Variations may be made tothe embodiments without departing from the spirit of the invention asclaimed. The above-described embodiments illustrate the scope of theinvention but do not restrict the scope of the invention.

What is claimed is:
 1. A thin film transistor comprising: a sourceelectrode; a drain electrode spaced from the source electrode; asemiconductor layer; a channel comprising a plurality of carbon nanotubewires, one end of each of the plurality of carbon nanotube wires isconnected to the source electrode, and an opposite end of each of theplurality of carbon nanotube wires is connected to the drain electrode,each of the plurality of carbon nanotube wires comprises a plurality ofsuccessively oriented carbon nanotube segments joined end-to-end by vander Waals attractive force, each of the plurality of the successivelyoriented carbon nanotube segments comprises a plurality of carbonnanotubes in contact with each other; and a gate electrode insulatedfrom the source electrode, the drain electrode, and the semiconductinglayer by an insulating layer.
 2. The thin film transistor of claim 1,wherein the plurality of carbon nanotube wires are substantiallyparallel to each other, and extend from the source electrode to thedrain electrode.
 3. The thin film transistor of claim 1, wherein adiameter of each of the plurality of carbon nanotube wires ranges fromabout 0.5 nanometers to about 100 micrometers.
 4. The thin filmtransistor of claim 1, wherein each of the plurality of carbon nanotubewires has a twisted structure.
 5. The thin film transistor of claim 1,wherein the plurality of carbon nanotubes are semiconducting carbonnanotubes.
 6. The thin film transistor of claim 1, wherein the pluralityof carbon nanotubes are selected from the group consisting of thesingle-walled carbon nanotubes, double-walled carbon nanotubes, andcombinations thereof.
 7. The thin film transistor of claim 1, wherein adiameter of each of the plurality of carbon nanotubes is less than 10nanometers.
 8. The thin film transistor of claim 1, wherein theinsulating layer is located between the semiconductor layer and the gateelectrode.
 9. The thin film transistor of claim 1, wherein theinsulating layer comprises of a material that is selected from the groupconsisting of silicon nitride, silicon dioxide, benzocyclobutene,polyester and acrylic resin.
 10. The thin film transistor of claim 1,wherein the source electrode, the drain electrode, and the gateelectrode comprise of at least one material selected from the groupconsisting of metal, alloy, indium tin oxide, antimony tin oxide, silverpaste, conductive polymer, and metallic carbon nanotube.
 11. The thinfilm transistor of claim 10, wherein the metal is selected from thegroup consisting of aluminum, copper, tungsten, molybdenum, gold,titanium, neodymium, palladium, cesium, and alloys thereof.
 12. The thinfilm transistor of claim 1, wherein the semiconductor layer is locatedon an insulating substrate, the source electrode and the drain electrodeare located on the semiconductor layer, the insulating layer is locatedon the semiconducting layer, and the gate electrode is located on theinsulating layer.
 13. The thin film transistor of claim 12, wherein amaterial of the insulating substrate is selected from the groupconsisting of silicon nitride, silicon dioxide, polyethyleneterephthalate, benzocyclobutenes, and acrylic resins.
 14. The thin filmtransistor of claim 1, wherein the gate electrode is located on aninsulating substrate, the insulating layer is located on the gateelectrode, the semiconducting layer is located on the insulating layer,the source electrode and the drain electrode are located on an surfaceof the semiconducting layer.
 15. The thin film transistor of claim 1,wherein the carrier mobility of the thin film transistor ranges fromabout 10 to about 1500 cm²/V⁻¹ s⁻¹, and on/off current ratio thereofranges from about 1×10² to about 1×10⁶.
 16. The thin film transistor ofclaim 1, wherein the semiconducting layer comprises of the channel, andthe channel extends between the source electrode and the drainelectrode.
 17. The thin film transistor of claim 1, wherein a length ofthe channel is in a range from about 1 micrometer to about 100micrometers, a width of the channel is in a range from about 1micrometer to about 1 millimeter, and a thickness of the channel is in arange from about 0.5 nanometers to about 100 micrometers.
 18. The thinfilm transistor of claim 1, wherein a distance between the plurality ofcarbon nanotube wires is less than about 1 millimeter.
 19. A thin filmtransistor comprising: a source electrode; a drain electrode spaced fromthe source electrode; a semiconducting layer connected to the sourceelectrode and the drain electrode; and a gate electrode insulated fromthe source electrode, the drain electrode, and the semiconducting layerby an insulating layer, wherein the semiconducting layer comprises aplurality of carbon nanotube wires, and at least some of the pluralityof carbon nanotube wires are connected to the source electrode and thedrain electrode, the plurality of carbon nanotube wires are crossed witheach other, each of the plurality of carbon nanotube wires comprises aplurality of successively oriented carbon nanotube segments joinedend-to-end by van der Waals attractive force, each of the plurality ofsuccessively oriented carbon nanotube segments comprises a plurality ofcarbon nanotubes in contact with each other.